1. Field of the Invention
The present invention relates generally to implantable, radially expandable medical prostheses including stents and stent-grafts. In particular, the present invention is a cobalt-chromium-molybdenum alloy stent and stent-graft.
2. Description of the Related Art
Medical prostheses frequently referred to as stents and stent-grafts are well known and commercially available. One type of stent, known as a self-expandable stent, is disclosed generally in the Wallsten U.S. Pat. No. 4,655,771, the Wallsten et al. U.S. Pat. No. 5,061,275, International Application Publication Number WO 94/24961, and International Application Publication Number WO 94/16646, all of which are hereby incorporated by reference in their entirety. These devices are used within body vessels of humans and other animals for a variety of medical applications including treating stenosis, maintaining openings in the urinary, biliary, esophageal and renal tracts, and vena cava filters to counter emboli.
Briefly, self-expanding stents of the type described in the above-identified patent documents are formed from a number of resilient filaments or elements which are helically wound and interwoven in a braidlike configuration. The stents assume a substantially tubular form in their unloaded or expanded state when they are not subjected to external forces. When subjected to inwardly-directed radial forces the stents are forced into a reduced-radius and extended-length loaded or compressed state. A delivery device which retains the stent in its compressed state is used to deliver the stent to a treatment site through vessels in the body. The flexible nature and reduced radius of the compressed stent enables it to be delivered through relatively small and curved vessels. After the stent is positioned at the treatment site the delivery device is actuated to release the stent, thereby allowing the stent to self-expand within the body vessel. The delivery device is then detached from the stent and removed from the patient. The stent remains in the vessel at the treatment site.
Materials commonly used for self-expanding stent filaments include Elgiloy.RTM. and Phynox.RTM. spring alloys. Elgiloy.RTM. alloy is available from Carpenter Technology Corporation of Reading Pa. Phynox.RTM. alloy is available from Metal Imphy of Imphy, France. Both of these metals are cobalt-based alloys which also include chromium, iron, nickel and molybdenum. Other materials used for self-expanding stent filaments are 316 stainless steel and MP35N alloy which are available from Carpenter Technology corporation and Latrobe Steel Company of Latrobe, Pa., and superelastic Nitinol nickel-titanium alloy which is available from Shape Memory Applications of Santa Clara, Calif.
The yield strength and modulus of elasticity of the filaments forming the self-expanding stent are important characteristics. The spring characteristics of an alloy and stents formed therefrom are determined to a large extent by the modulus of elasticity of the alloy. In general, the modulus of elasticity must be high enough to allow the stent to spring back toward its unloaded state from the compressed state with sufficient radial force to meet the needs of the application for which the stent is designed. The material must also have sufficient strength that it can be compressed for delivery without being plastically deformed or permanently bent. Elgiloy.RTM., Phynox.RTM., MP35N and stainless steel are all high strength and high modulus metals. Nitinol has a relatively lower strength and modulus.
Elgiloy.RTM., Phynox.RTM., MP35N, Nitinol and stainless steel alloys all contain about 10%-20% nickel. Nickel enhances the ductility of the alloys, improving its ability to be mechanically drawn or formed (i.e., reduced in cross-sectional area) into wire of the relatively fine diameters required for stents (between about 0.025 mm and 0.500 mm) by a process known as cold working. Cold working is also desirable because it increases the strength of the material. However, the yield strength that can be obtained by cold working Elgiloy.RTM., Phynox.RTM., MP35N, Nitinol and stainless steel alloys (e.g., about 1738 MPa (252) ksi for Elgiloy.RTM. alloy) is generally not high enough for many stent applications. As a result, stents fabricated from the Elgiloy.RTM. and Phynox.RTM. cold worked (also known as wrought) alloys are typically heat-treated after they are cold worked, a process that significantly increases their yield strength and thereby allows for the fabrication of stents with relatively smaller diameter filaments. By way of example, the yield strength of Elgiloy.RTM. alloy can be increased by heat treating to about 2861 MPa (415) ksi. The strength of stainless steel alloys and Nitinol cannot be significantly increased by heat treatment, so these materials are typically not used in the construction of self-expanding stents with high radial strength.
Cold working is a method by which metal is plastically deformed into a particular shape and work (strain) hardened to increase the strength of the material. Processes that can be performed to accomplish cold working are drawing, rolling, extruding, forging, swaging, and the like. Raw material is input into the cold working process in the form of ingots, rods, bars, billets, blanks, or other appropriate shapes. The workpieces are forced to pass through a die, fill a die cavity, or conform to the shape of a die. The output of the cold working process is typically material with a new form and with higher strength and hardness from the metallurgical strain hardening that occurred with the plastic deformation. In a cold working process described in International Publication Number WO 94/16646, billet, bar, rod, or wire is drawn or extruded through a series of round dies and incremental reduction in the material diameter is achieved until the final desired wire size is obtained for stent braiding.
The filaments of the stents described above may form a lattice structure which includes large amounts of open area. In some cases, however, this large open area allows tissue to grow through the stent and occlude portions of the tract that were opened by the stent. For applications where tissue ingrowth of this type is undesirable, as well as for applications in which portions of the tract being treated are weak or have gaps (e.g., aneurysms), it is generally known to use covered stents. Stents or stent-grafts may be covered, for example, by porous membranes, interwoven organic filaments, or the like. Stents of this type are sometimes known as covered stents or stent-grafts, and are disclosed, for example, in Experimental Assessment of Newly Devised Trans-Catheter Stent-Graf for Aoritic Dissection, Annual of Thoracic Surgery, M. Kato et al., 59: 908-915 (1995). The membranes incorporated into the stent-grafts are typically formed of polymeric materials. However, many of these polymeric materials can degrade when exposed to temperatures used to heat-treat alloys of the type described above. The need to heat-treat the metal alloy lattice structure and the temperature sensitivities of the polymers used to form the membranes therefore constrain stent-graft designs and their application.
In addition to drawn elongated filaments for interwoven element stents of the type described above and in the Wallsten U.S. Pat. No. 4,655,771, metal alloy materials are drawn or extruded into other forms for stent fabrication. The Palmaz U.S. Pat. No. 4,733,665 relates to a stent fabricated from a drawn or extruded stainless steel tube. The Gianturco U.S. Pat. No. 4,800,882 relates to a stent assembled from a drawn stainless steel wire. Other known stents are fabricated from drawn, extruded, or rolled nickel-titanium alloy ribbon.
Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys have been used in medical implant applications. Chemical, mechanical, and metallurgical requirements for alloys of these types used for surgical implant applications are published in ASTM Standard Designations F 75 and F 799. One such alloy known as BioDur Carpenter CCM.RTM. is commercially available from Carpenter Technology Corporation. These chromium-cobalt-molybdenum alloys are highly biocompatible. However, since they have a relatively low nickel content (about 1% maximum), cobalt-chromium-molybdenum alloys have relatively low ductilities and high work hardening rates that limit their formability. For this reason the conventional wisdom has been that these alloys cannot be cold drawn down to the fine wire diameters needed for stents and stent-grafts.
There remains a continuing need for improved stents and stent-grafts. In particular, there is a need for stents and stent-grafts fabricated from highly biocompatible alloys having high yield strengths and high moduli of elasticity. There is also a need for stents and stent-grafts that do not require heat treatment.